The present invention relates to a hydraulic valve, and especially to a hydraulic valve having a low static flow force.
Hydraulic transmissions has many advantages relative to other types of transmissions such as mechanical transmissions and the like, such as small weight, sensitive to operations, capable of being frequently started and reversed, easy to operate and control and the like. The hydraulic transmissions thus have been widely used in a greater number of mechanical apparatuses.
A hydraulic control valve is indispensable to the various hydraulic transmissions, and is an element that controls the fluid pressure, flow rate and direction in the hydraulic transmission, and slide valve is a type of common hydraulic control valve.
To accurately control the slide valve, profound researches should be made on the stress and working process of the slide valve. Normally the hydraulic force, thrust force and spring force exerted on the slide valve are controllable and predictable. However, when the hydraulic fluid flows through the valve port, the spool will be suffered from an extra acting force, i.e., a flow force, since the change in the flow direction and flow rate will vary hydraulic fluid momentum, and such a force in turn will have an effect on control of the spool. The flow force normally varies with the size of the valve port, the flow amount through it and inlet pressure and so on.
The flow force may comprises a static flow force and a transient flow force, and the so-called static flow force refers to a force that acts on the spool and causes the valve to tend to close due to the change in momentum when the hydraulic fluid passes through the valve port, in terms of the certain valve opening amount. Thus, the static flow force will increase the force required for operating the slide valve, and especially in the case of high pressure and great flow rate, it has the significantly adverse influence on manipulation of the slide valve, even leading to the situation that the flow force is larger than the manipulation force to make action of the slide valve invalid. Thus, the problem as how to reduce or counteract the static flow force has received much attention in the hydraulic field.
Currently, the main solutions are as follows: 1), configuring the valve chamber in such a way that the hydraulic fluid flowing into and flow out from the valve chamber has the unchanged axial momentum, so as to reduce the axial flow force; and 2), employing a pilot valve for controlling the main spool. However, each solution will complicate the casting and machining processes of the slide valve, and further an extra pilot valve may be required, thereby increasing the cost of the slide valve. At present, the design for the valve chamber is relatively complex, causing the inconveniency to machine and manufacture.
Thus, it is urgent to provide a simple design for reducing the flow force of the slide valve. Reduction in the static flow force will in turn further simplify the design of the slide valve and reduce its cost.